1. Field of the Invention
The present invention relates to air traffic control ("ATC") radar systems, and more particularly to a data processing system for an air traffic control radar system.
2. Description of Related Art
An example of air traffic control tracking algorithms currently in use is the Mode Select Beacon System Sensor (MODE-S). MODE-S combines target responses produced by a primary radar with the target responses produced by a secondary radar. The primary target responses depend upon a reflection of a radar pulse of short duration from, for example, an aircraft wherein the range of the aircraft is determined from the round-trip time of the radar pulse. The secondary target data rely upon a response from a transponder or beacon located in the aircraft upon interrogation of the aircraft by a transmitted radar pulse transmitted by the ground-based antenna. The transponder response may indicate range, bearing, altitude, and identity of such aircraft.
The radar antenna can be mechanically or electronically scanned and typically focuses the radar pulses in a fairly narrow angular beam. A system incorporating an electronically-scanned antenna is disclosed, for example, in U.S. Pat. No. 5,825,322. A transmitter controlled by data processing electronics typically controls the signal strength and the duration of the pulses emitted by the antenna. The antenna commonly also serves as a receiver for returning signals, which are received when the antenna points at a target, but may also include superimposed interference signals due to noise. Data processing electronics processes the received signals using known processing algorithms to extract, for example, the position, velocity, direction of motion, and the type of target.
Before describing the various functions performed by the radar system in greater detail, it will be useful to define some of the terminology to be used: "Target data" are data points supplied by a suitable output of the radar acquisition system (having a primary and/or secondary antenna) and commonly expressed in polar coordinates (r, .THETA.) which are most suited for display. A "target report" is an association of target data (r, .THETA.) with a specific object (target). A "target track" is a time-sequence of target reports that have a high probability of being correlated and may be visually displayed indicating a "course" of the target. "Correlation" is a process used to establish a connection between multiple target reports. "Search acquisition time" is the time required to scan a wedge-shaped area bounded by a predetermined arc and a predetermined distance from the radar site. A "search radius" is the radius of a circular area surrounding a target in which the target is expected to be located in a subsequent radar scan. "Process delay time" refers to the time required to organize and buffer incoming and outgoing data. The buffering process typically introduces "jitter" in the target data causing a time spread in the arrival time of the data at the STARS processor. The "Dwell delay" is defined as the sum of the "jitter" due to process delay of incoming data (see above) and the time needed for the radar to scan all potential targets within the search radius of a target being tracked.
Efforts to modernize air traffic computers, displays and computer software are being undertaken in the STARS program of the Federal Aviation Administration ("FAA"). The STARS processing system is subject to various regulatory and technical requirements. One aspect of the of the STARS system requirements relates to data processing times; that is, to satisfy STARS requirements, certain specified data processing tasks may not exceed certain predefined time intervals. These time intervals are specified by statistical measures requiring, for example, that 95% of the targets are properly identified and correlated with a target track within 1 second for radar scenarios with a mixture of primary and secondary received target data.
Systems processing target data must provide reliable results within the specified data processing time even under challenging conditions, such as garble, malfunction or a total failure of a secondary radar site. In such an event, a controller must be able to rely on information received from the primary radar which lacks the identity code of the target otherwise supplied by the airborne transponder or beacon. Beacon reports from airborne transponders may be garbled in high traffic areas (target bunching), making it difficult to compile a reliable target report and ultimately a target track. Targets most likely to be misidentified are those having a relatively high angular velocity relative to the antenna, such as targets moving at high speed and/or targets located close to the radar site. Also, target tracks crossing at a shallow angle may be subject to swapping of identities.
Currently, radar sites relying on primary antennas for target tracking use a fixed dwell time. The dwell time has to be long enough so that multiple target data within the search area can be considered before a decision is made on which target report to correlate with which target track. For example, a conventional radar tracking system may typically use a fixed dwell time of 450 ms and attempts to correlate target data with a target track every 150 ms, hereinafter to be referred to as a time "quantum." Using a fixed dwell time for all target ranges, for short- and long- range radars, and for radars with different antenna periods is a compromise, since the search acquisition time for a defined search radius depends on the distance of the target from the antenna, and the antenna period. For example, for a target with a high angular velocity (the angular velocity is defined as the component of the linear velocity of the target perpendicular to the radius connecting the target and the radar site, divided by the distance of the target from the radar site), the search acquisition time may be 0.5 second so that the fixed dwell time of 450 ms is insufficient. Conversely, a search acquisition time of only 50 ms may be sufficient for distant targets which can therefore be correlated with a target track within a much shorter time.
A radar processing system processing target data with a constant 450 ms dwell time for all situations therefore appears to be inefficient and makes it difficult to satisfy a mandated one second response time for processing, correlating and displaying STARS target data as target tracks in 95% of the acquired target samples under all conditions.
Thus, a need exists for data processing methods and systems that reliably correlates target data with a target report and a target track within a shorter time, while satisfying overall system requirements for data processing speed, without increasing the frequency of correlation errors or requiring replacement of conventional mechanically scanned antennas.